Generally, a radio transmission device modulates and transmits information bit to be transmitted. Examples of a modulation scheme used for modulation of information bit include a binary phase shift keying (BPSK), a quadrature phase shift keying (QPSK), a 16 quadrature amplitude modulation (QAM), and a 64QAM. A modulation multi-value number refers to the number of bits to be transmitted per symbol. For example, in the BPSK scheme, one bit can be transmitted per symbol, and in the QPSK scheme, two bits are transmitted per symbol. Further, in the 16QAM scheme, four bits can be transmitted per symbol, and in the 64QAM scheme, six bits can be transmitted per symbol.
In modulation performed by the modulation schemes, mapping according to a constellation representing signal points obtained by plotting an in-phase component (hereinafter, “I component”) and a quadrature component (hereinafter, referred to as “Q component”) of a signal on an in-phase quadrature (IQ) plane is performed. That is, when information bit is modulated by each modulation scheme, bits corresponding to a modulation multi-value number are mapped to a symbol corresponding to any one signal point on the constellation. Specifically, a 16QAM constellation is formed, for example, as illustrated in FIG. 1. In the 16QAM constellation, 16 signal points are arranged, and each signal point corresponds to one symbol that corresponds to a combination of four bits. For example, when four bits “0000” are modulated by the 16QAM scheme, a symbol corresponding to signal point closest to an original point among four signal points present on a first quadrant (a quadrant in which both of the I component and the Q component are positive) is generated.
The constellation becomes different according to a modulation scheme. As the modulation multi-value number increases, the number of signal points to be arranged increases, and thus the distance between signal points decreases. If a phase or an amplitude of a symbol changes due to influence of noise or fading at the time of transmission, the position of a signal point corresponding to a symbol changes. As the modulation multi-value number of the modulation scheme increases, demodulation errors occur more easily. Thus, in a state in which a propagation environment is good, if the modulation multi-value number increases, since the number of bits to be transmitted per symbol increases, transmission efficiency is improved. However, in a state in which a propagation environment is bad, if the modulation multi-value number increases, since demodulation is not correctly performed, retransmission is frequently performed, thereby deteriorating transmission efficiency. For this reason, a study on modulating data by a modulation scheme in which a constellation at the time of first transmission is different from a constellation at the time of retransmission, and thus improving the whole throughput has been conducted.
Meanwhile, hierarchical modulation of hierarchizing and modulating a plurality of bits per symbol has recently attracted attention. For example, in the hierarchical modulation, four bits are hierarchized into upper-order two bits and lower-order two bits and symbol-mapped so that an error can become difficult to occur in the upper-order two bits compared to the lower-order two bits. For example, in the constellation illustrated in FIG. 1, upper-order two bits including a first bit and a second bit mapped to each signal point are all “00” on four signal points on the first quadrant and all “10” on four signal points on a second quadrant (a quadrant in which the I component is negative and the Q component is positive). Similarly, upper-order two bits are all “11” on four signal points on a third quadrant (a quadrant in which both the I component and the Q component are negative) and all “01” on four signal points on a fourth quadrant (a quadrant in which the I component is positive and the Q component is negative).
Thus, even if the position of the signal point on the constellation changes due to influence of noise or fading, the upper-order two bits can correctly be demodulated unless an I axis or a Q axis is straddled due to that change. That is, in the 16 QAM constellation illustrated in FIG. 1, since the average distance on the upper-order two bits is large, the upper-order two bits of the four bits mapped to each signal point become relatively difficult to cause an error.
However, third bits in FIG. 1 are all “0” on signal points of two rows close to the I axis among signal points of four rows arranged in a direction of the I axis and are all “1” on signal points of two rows far from the I axis. Similarly, four bits in FIG. 1 are all “0” on signal points of two rows close to the Q axis among signal points of four rows arranged in a direction of the Q axis and are all “1” on signal points of two rows far from the Q axis. Thus, if the position of the signal point on the constellation changes due to influence of fading, even when a change corresponding to half of the distance between signal points occurs, a demodulation error occurs in the lower-order two bits. That is, in the 16QAM constellation illustrated in FIG. 1, since the average distance between the signal points on the lower-order two bits is small, an error occurs relatively easily in the lower-order two bits among the four bits mapped to each signal point.
However, in the conventional modulation scheme using the constellation, there is a problem in that an error resilience is not greatly different between the upper-order bits and the lower-order bits, and a sufficient degree of accuracy of demodulation is not obtained. That is, in the constellation in FIG. 1, since a signal point corresponding to four bits “0011” is far from the I axis and the Q axis, even if a phase or an amplitude greatly changes, a demodulation error does not occur in the upper-order two bits. However, since a signal point corresponding to four bits “0000” is close to the I axis and the Q axis, the position of the signal point is more likely to change while straddling the I axis or the Q axis due to a change in phase or amplitude, and thus there is little difference of an error resilience between the upper-order two bits and the lower-order two bits. That is, the upper-order bits are almost the same in error rate as the lower-order bits, and a degree of accuracy of demodulation of a symbol is not sufficiently improved.
Further, in order to sufficiently differentiate the upper-order bits from the lower-order bits, instead of arranging the signal points at a regular interval, the signal points inside each quadrant may be arranged away from the original point of the IQ plane. However, if the signal points are away from the original point in general, a maximum amplitude of a symbol increases, and thus maximum transmission power increases. Further, if the position of a signal point farthest from the original point in each quadrant is fixed and the other signal points are arranged away from the original point, although maximum transmission power does not change, average transmission power increases, and a degree of accuracy of demodulation on the lower-order bits deteriorates.
Patent Document: Japanese National Publication of International Patent Application No. 2005-533461